To reduce fuel consumption and CO2 emissions different concepts are implemented in modern motor vehicles which improve the internal combustion engine efficiency, the exhaust cleaning or the aerodynamics of the motor vehicle. One of these concepts for reducing the fuel consumption is what is known as stop/start operation. In this mode of operation the internal combustion engine is switched off during longer periods when the vehicle is stationary. Since, especially with otto engines, the efficiency of the internal combustion engine is greatly reduced when idling, considerable savings in fuel consumption can be obtained in this way, for example in the urban driving cycle.
In a known method for stop/start operation of a motor vehicle the internal combustion engine is switched off if the motor vehicle is idling for a specific time. The internal combustion engine is switched off automatically by switching off the fuel injection. After the switch-off of the fuel feed, because of the inertia of the crankshaft and the piston, it takes a certain time, typically between 500 and 2000 ms, until the engine speed has dropped from idling speed to zero. Problems arise if the driver of the vehicle wishes to run the internal combustion again during this switch-off phase. To do this torque must be built up by restarting the fuel feed and with otto engines by restarting the ignition. This however involves significant control effort for correctly dispensing the quantity of fuel to be fed as well as for correct synchronization of the fuel injection with the ignition. If too much fuel is supplied, the result can be wall film formation and thereby an increased emission of pollutants. If too little fuel is supplied, the torque build-up is inadequate and the internal combustion engine can stall. Similar problems arise with inadequate synchronization of the fuel injection and the ignition. Establishing sufficient torque is additionally made difficult by the fact that the speed on switch-off of the fuel feed falls with a very steep gradient. All these problems result in the establishment of a sufficient torque during of the switch-off of the internal combustion engine only being possible down to speeds just below the idling speed. If the speed of the internal combustion engine has fallen too far, the internal combustion engine can only be started again from its completely stopped state with the aid of the starter. Because of the high control overhead this method involves the danger of a delayed start behavior in stop/start operation and an increase in pollutant emissions during restart.